The application relates generally to novel perylene dye compounds and, more specifically, to such dye compounds which are useful in liquid crystal guest-host applications and to liquid crystal compositions and devices.
It is known in the art to use dyes, including perylene dyes, in conjunction with liquid crystalline materials to obtain improved color displays in liquid crystal display devices. In liquid crystal guest-host systems, a "guest" dye is dissolved in the "host" liquid image-forming medium in the display device. The guest dyes, which may be used alone or in combination with other dyes to achieve a desired color for the display, desirably should possess properties such as dichroism, high extinction coefficient, relatively high solubility in the liquid crystal material and high order parameter.
The solubility of the dye(s) in the liquid crystal material must be such that the relatively thin liquid crystal layer in the display device possesses adequate absorption in one of the oriented states of the dye(s). Order parameter is a quantitative measure of the degree of molecular order or alignment in a given system. High order parameter is promoted by dyes with elongated shape having a large ratio of molecular length to width, similar to the shape of the molecules of the liquid crystal host material. To insure an elongated shape the molecules should also have a rigid structure.
The brightness and contrast of a liquid crystal display device are both related to the order parameter, normally designated S, of the dye, where S=(A.sub..parallel. -A.sub..perp.)/(A.sub..parallel. +2A.sub..perp.), A.sub..parallel. is the light absorption of the dye measured with a polarizer parallel to the nematic director of the liquid crystal host at the wavelength of maximum absorption and A.sub..perp. is the light absorption of the dye measured with the polarizer perpendicular to the nematic director. Desirably, the order parameter of the dyes should be as high as possible for example, at least 0.60 and preferably higher, in order to achieve a desired contrast ratio while at the same time allowing the fabrication of a liquid crystal display device having a reasonable brightness parameter. Thus, there is a continuing need in the art for dyes which are soluble in liquid crystal materials, at least in the amount typically employed to provide a suitable display device, e.g., up to about 1% or more, and which have a relatively high order parameter.